17.1 Reducing Signal Drift

17.1 Reducing Signal Drift

Before we get into cracking Wired Equivalent Privacy (WEP) and discuss possible countermeasures, let us pause to consider how the humble antenna can help control radio frequency signal drift. Antennas can be used for both good and evil. On the one hand, you can control the signal drift of your wireless LAN (WLAN) by manipulating antennas. On the other hand, directional antennas make it easier for wardrivers to probe your networks from a distance.

For example, a wardriver can use a mobile 2.4-GHz antenna from her car parked down the street to boost the signal bleeding from your house. To counter this to some extent, you can position your access point (AP) antennas to point away from the street. You can also move the access point to the center of your house to reduce signal bleed . You can even reduce (or turn off) the signal on one or both of your AP antennas using the software that ships with most quality access points.

On the enterprise side, you can also use directional antennas to focus your signal. For example, we recently set up a long-distance building-to-building link. To do this we used a 24-dB parabolic antenna on the transmitting side (Figure 17-1). The goal was to achieve a strong link over a long distance, while avoiding excessive signal scatter.

Figure 17-1. Our parabolic antenna shown in horizontal polarization; in suburban terrain, mounting in vertical polarization produces less signal scatter than horizontal polarization

We bought this high- powered antenna on eBay for less than $50. As you can see, this particular antenna is quite large. Thus, you must have adequate room for mounting (you need to do a rooftop mount, rather than a wall side-mount). Otherwise, you should select a more slender Yagi antenna. You can also build your own directional antenna out of a Pringles ¢ can.

The 24-dB antenna in Figure 17-1 has a very tight beam width of only eight degrees. This helps prevent signal bleed along the transmit path . However, be careful, as you can still get some signal bleed behind the antenna, to the sides, and especially past your target (overshoot). By using antenna positioning, directional antennas, and power output tweaks, you can help prevent excessive signal bleed. This provides a modicum of additional security, but of course is only a small part of your total security solution. We discuss other ways to protect your transmissions later in the chapter.